New
observations from the Japanese SELENE (Kaguya) lunar mission are shedding light
on the perplexing geological differences between the near and far sides of the moon.
For now
there are no breakthroughs, but the new data is expected to provide researchers
with lots of clues.
Not until
the dawn of the Space Age were humans able to get a glimpse at the moon's far
side, which is perpetually kept from Earth's view by the satellite's synchronous
rotation — it rotates about its axis in about the same time it takes it to
orbit the Earth.
Once
spacecraft were launched that could take in the long-hidden view, scientists
found that the moon was two-faced: the nearside was covered with smooth, dark volcanic maria
(solidified pools of ancient lava), while the far side almost completely lacks
these features and is instead covered by more heavily cratered bright material.
These
differences in topography and composition imply that the two sides evolved
differently during the more than 4 billion years of the moon's existence.
SELENE's
instruments have provided the most detailed topographic map of the moon to
date, as well as measurements of the variations of gravity across its surface
and a look at what lies beneath. The observations
are detailed in a series of studies presented in the Feb. 13 issue of the
journal Science.
Different
paths
The
prevailing theory of how
the moon formed and evolved is that a Mars-sized body collided with the
Earth shortly after the solar system began to form about 4.5 billion years ago
and rent out a chunk from the Earth's mantle that eventually fell into orbit
around Earth and cooled over millions of years, coalescing into the moon.
The moon
was initially covered by a deep magma ocean that gradually cooled and hardened
into crust. Space rocks continuously bombarded the lunar surface until about
3.8 billion years ago.
The decay
of some elements in the lunar mantle produced heat that melted the surrounding
rock and fueled volcanism that formed the maria (which means "seas") that
appear as dark splotches on the lunar surface.
Some type
of imbalance arose that caused the near and far sides to develop differently,
with far more maria on the near side. Whether that imbalance was the result of
a giant impact or some internal change wasn't known.
The SELENE
results have helped resolve that issue.
Harder,
cooler
SELENE was
able to measure the gravity anomalies across the far side of the moon, and
researchers could compare then to those of the near side.
The
gravitational patterns of the far side confirmed that it had a harder, cooler
lithosphere (the outermost shell of any rocky moon or planet) than did the near
side during the period of major impacts, said Gregory Neumann of NASA's Goddard
Space Flight Center in Greenbelt, Md. and who is not part of the SELENE team.
Having a
hard, cool lithosphere that far back in the history of the moon points to an
internal cause as the most likely differentiating factor that caused one side
to be warmer and more pliable than the other.
The
spacecraft also used its laser altimeter to make the most highly-resolved
topographic map of the moon to date.
"It's
stunning," Neumann told Space.com.
The map
also suggested that the crust on the far side was rigid and may therefore lack
water and other evaporating compounds, which are part of what makes Earth's
crust so bendable.
Early
volcanism
SELENE's
observations also shed some light on volcanism in the moon's early history. A
radar instrument that can penetrate the lunar surface indicated that there were
debris layers between some of the volcanic basalt flows, which suggests that
the volcanism stopped and started a bit in the moon's early history.
A fourth
study, also detailed
earlier in an online version of Science, suggested that volcanism on
the far side of the moon lasted longer than previously thought, though not as
long as it did on the near side.
Modelers
will be able to use all of this data gleaned from the SELENE studies to build a
better picture of how the moon formed and evolved.
"The
modelers are going to have a field day with this," Neumann said.
Though this
SELENE data adds to the picture of the geology of the moon, "we still
haven't got any huge headlines out of the moon," such as ice
hidden in lunar craters, Neumann said.
But other
missions, such as China's Chang'e-1 and India's Chandrayaan-1, and future
missions, such as the upcoming NASA Lunar
Reconnaissance Orbiter mission (of which Neumann is a team member), will
all shed more light on enigmatic history of our closest planetary neighbor,
Neumann said.
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